A properly operating catalytic converter dampens oxygen concentration fluctuations in the exhaust stream. A system proposed in SAE paper No. 900062, Detection of Catalyst Performance Using On-Board Diagnostics, employs two exhaust gas oxygen sensors, one upstream and one downstream of the catalytic converter, to detect oxygen content in the exhaust gas. The system employs test signals in the form of an air/fuel ratio swing on both sides of stoichiometry at predetermined rates or frequencies caused by fuel control system perturbations. A determination of catalytic converter efficacy is said to be made by comparing the difference in response patterns between the upstream and downstream EGO sensors. In such a system, the monitoring results depend on signals from two EGO sensors which may have different characteristics, due either to manufacturing tolerances or to differential aging over a period of use.
A number of systems are known which attempt to determine catalytic converter efficacy based largely on measures of the oxygen storage capability of the converter. A converter with reduced oxygen storage capability may, however, nevertheless function with acceptable efficiency in treating exhaust gas. That is, the treated exhaust gas may meet emission requirements notwithstanding such reduced oxygen storage capability, particularly by effective use of oxygen components present in the exhaust gas. In such cases, an accurate catalytic converter efficiency test could be less directly related to oxygen storage capability. It is particularly desirable to provide a monitoring system and method in a fuel control system.